This invention relates to inhibition of liquid and vapor transmission through a seal between a substrate, such as a metal or glass, and a plastic resin. Further, this invention relates to a process that inhibits liquid and vapor transmission through such seals and to specific articles formed using this process. More particularly, this invention relates to a powder coated metal object which is enclosed by a molded plastic unit and more specifically to a metal object that is powder coated with a polyolefin or a polyolefin-polycarbonamide blend.
The problem of liquid and vapor transmission through a metal plastic seal is exhibited in lead-acid batteries commonly used in motor vehicles. In one method of making lids which are used in the outer casing of such batteries, polypropylene is injection molded about lead terminal bushings which are positioned in the lid such that lead positive and negative terminal posts extending from the battery plates pass through the bushing during assembly of the battery. The bushings and posts are secured together by a lead on lead seal. After the assembled battery is charged with battery acid, the liquid and vapor can travel up a post by surface forces (wicking) and eventually can make contact with the seal between the polypropylene lid and the lead bushing. Often seepage occurs between the bushing and the lid, which leads to an unsightly corrosion about the battery terminals. Since this seepage and resulting corrosion often occur when the filled batteries are stored prior to sale, such affected batteries become undesirable to purchasers.
One partial solution to the battery wicking problem is a Bi-Wax cup fitted onto the bushing underneath the lid. Such a device, a short tube with one end partially closed, is attached to the bushing using a greasy lubricant. Although acid wicking is prevented, this method is too expensive to be used commercially.